Miniature x-ray tubes have been used extensively in portable equipment for non-destructive analysis, material characterization, imaging and medical applications. The portability of the instrumentation and point of use nature of the resulting systems demands that they be able to make measurements rapidly and consistently under a range of operating parameters. One of the key aspects of this type of operation is the ability of the x-ray tube to begin generating stable output tube current as rapidly as possible.
To date, miniature x-ray tubes have utilized primarily analog circuitry to control the filament of the x-ray tube. The basic problem with the control system is that the transfer function gain increases dramatically as the requested tube current increases. The highest open loop gain occurs at the maximum allowable operating tube current for the x-ray tube. For stable operation using an analog control circuit, the overall gain needs to be set to ensure stability at this maximum gain. Optimization for maximum current gain allows for fast turn-on and settling time with minimum overshoot of the tube current when running at maximum tube current.
The issue with the analog circuit implementation is that when requesting lower tube currents, the gain is much lower than in maximum current operation resulting in excessive turn-on and settling times. These increases in turn-on and settling time can result in incorrect measurements or increased assay times for proper results.